Space weathering observed on the samples returned from Ryugu

The samples returned from the near-Earth asteroid (162173) Ryugu by the Hayabusa2 spacecraft provide the first opportunity for laboratory study of space weathering signatures on the most abundant C-type asteroids. Many (about 60%) of them are thought to have experienced high degrees of the aqueous alteration as shown in CI and CM carbonaceous chondrites. Hayabusa2 measured in situ near-infrared reflectance spectra of Ryugu using the NIRS3 spectrometer. The shallow 2.7-µm absorption band in the spectra was interpreted as a signature of thermal metamorphism or solar radiation heating. However, all the investigations of Ryugu grains performed to date show that the Ryugu materials are genetically common to CI chondrites. The Ryugu grains are abundant in phyllosilicates (saponite and serpentine), magnetite, Fe-Ni sulfides, and carbonates. This study about the space weathering of Ryugu grains explains this discrepancy. Ryugu is exposed to the major agents of space weathering of airless bodies. However, the resultant space-weathering products are substantially different from that of the Moon and Itokawa, both of which are composed of anhydrous minerals. Weathered Ryugu grains show areas of surface amorphization and partial melting of phyllosilicates, in which reduction from Fe³⁺ to Fe²⁺ and dehydration developed. Comparison of space-weathered materials with the run products of a helium irradiation experiment on non-space-weathered grains and of laser irradiation experiments of Murchison CM chondrite shows that the amorphization of phyllosilicates may be caused by solar wind irradiation and the partial melting, by micrometeoroid impact heating. Space weathering likely contributed to dehydration by dehydroxylation of Ryugu surface phyllosilicates that had already lost interlayer water molecules and to the weakening of the 2.7-µm hydroxyl (–OH) band in reflectance spectra. For C-type asteroids in general, this indicates that a weak 2.7-µm band can signify space weathering-induced surface dehydration, rather than bulk volatile loss.